Transformers equipped with tap changers are frequently used in different types of power transmission environments, such as at 10 kV and above. A transformer that comprises a tap changer is able to change the turns ratio between the windings to thereby change voltage levels. This ability is in many systems used for controlling the delivery of power.
Transformers are generally reliable. The probability of them failing is low, such as around 1%. However, of those 1% that fail, typically 20-40% are due to failure in the tap changer.
The reason for this is that the tap changer is the only part of the transformer that has mechanically moving elements. Therefore this part of the transformer is more likely to cause a failure than the rest of the transformer.
When monitoring a transformer it would therefore be of interest to monitor a tap change operation when the turns ratio is changed. If such monitoring is performed then it may be possible to predict when the tap changer is about to wear out. Thereby maintenance may be more easily planned, which is of advantage both with regard to reliability and economy.
Tap changer monitoring is described in a number of documents.
US 2012/0173180 does for instance disclose a method and apparatus for evaluating health of a tab changer. The health is evaluated through separating a second discharge signal from a first discharge signal using a third discharge signal and determining the health through comparing the second discharge signal and a first initial value. The third discharge signal may be a second initial value or a fourth discharge signal. The first initial value is a signal generated in the tab changer and the second initial value is a signal generated in a main tank of the transformer, while the first and fourth discharge signals are high frequency currents in a grounded line of the tab changer. The document also mentions the use of an output voltage detector detecting the output voltage of a secondary coil and used in the control of the tab changer.
EP 2541 572 discloses a protection arrangement for a tap changer where the current through the tap changer is detected and used to generate a current indication signal. The duration of the current indication signals is then compared with a threshold and a fault indication signal generated if it is.
JPH 08-213257 is concerned with obtaining a measure of the health of a tap change based on a current signal corresponding to the current through the tap changer.
JP 2000-208340 is concerned with using a waveform obtained through measuring power transmission properties on one side of a transformer.
However, a tap change is not easily detected in the waveforms that are available for monitoring purposes. This is especially the case when the transformer is used at high loads. Furthermore, it is often not enough to merely detect a tap change in a waveform. It may be necessary to also extract information indicative of the health of the tap changer from such waveforms. It can thereby be seen that it may be very hard to determine the health of a transformer using the available waveforms.
It is therefore of interest to monitor a transformer equipped with a tap changer in a way that information indicating the performance of the tap changer can be extracted using the available waveforms and used for condition monitoring purposes.
The present invention is directed towards such transformer monitoring.